Chapter5 Theory

Four stroke cycle theory

Intake strokePiston moving downIntake valve openExhaust valve closed

Copyright 2003 Gary Lewis - Dave Capitolo

Four stroke cycle theory

Compression strokePiston moving upIntake valve closedExhaust valve closed

Four stroke cycle theory

Power strokePiston moving downIntake valve closedExhaust valve closed

Four stroke cycle theory

Exhaust strokePiston moving upIntake valve closedExhaust valve open

Four stroke cycle theory

Each stroke takes 180° of crankshaft rotation to complete

All cylinders fire in 720° of crankshaft rotation

720 divided by number of cylinders = firing intervalOdd fire V-6 engine (90° block with 120° rod journals)

Four stroke diesel theory

Compression ignitionDiesel fuel low volatilityHigh compression ratios produce the heat necessaryPre-chamber for vaporization

Rotary engine theory

Piston dwell time

Piston travel is at a minimum. . . TDC and BDCCrank moves horizontally

Piston velocityMaximum when rod is 90° to crank

AccelerationMaximum 30° earlier

Best VE is obtained by synchronizing valve openingwith piston speeds

Other engine cycles

OverlapBoth valves are openEnd of exhaust & start of intakeLow pressure in exhaust port

BlowdownExhaust valve opens before BDCTo help evacuate cylinder before piston reversesPumping losses at end of exhaust stroke

Valve events

Intake valve openingBTDCLow pressure in cylinder

Intake valve closingABDCCylinder pressure is effected by timing

Exhaust valve openingBBDCResidual pressure helps blowdown

Exhaust valve closingATDCLow pressure in exhaust port draws air in

Effects on valve timing

Intake valve openingLate – Reduced VEEarly – Dilution of intake with exhaust

Intake valve closingLate – Reduces cylinder pressureEarly – Increases cylinder pressure

Exhaust valve openingLate – Pumping lossesEarly – Power reduction

Exhaust valve closingLate – Reduces vacuumEarly – Reduces VE

Combustion

Spark ignitionMaximum cylinder pressure 15° ATDCTumble and swirlMotion reduces misfiresExcess motion inhibits flowAFR 14.7:1 at part throttle, 12.5:1 under load

Compression ignition18:1 direct injection23:1 pre-chambers for better startingCranking heats air to 600°FCompression heats are to 800-1200 °F

Diesel fuels

Cetane volatility numbers 50-55Higher cetane #1 fuel for cold weatherLower cetane #2 fuel for warm weatherParaffin separates from fuel at 20°F

Valve trains

OHV (overhead valve)Pushrod configurationMany reciprocating partsHigher valve spring pressure requiredCompact engine size compared to OHC

Valve trains

OHC (overhead cam)Fewer reciprocating partsReduced valve spring pressure requiredHigher RPM capabilityCylinder head assemblies are taller

Valve trains

Cam-in-headNo pushrodsUse rocker arms

Valve lash compensators

Solid liftersNo internal partsPeriodic adjustment

Valve lash compensators

Hydraulic liftersTo maintain zero lashQuieterNo periodic adjustmentAnti-scuff additives are required in oils

Hydraulic lifter operation

Valve closed• Oil flows through lifter bore, &

past check valve• Plunger return spring maintains

zero lash

Hydraulic lifter operation

Valve open• Check valve seats and limits the slippage• Now operates as a solid lifter

Hydraulic lifter operation

Return to valve closed• New oil enters the lifter body• This oil replaces oil that has leaked between plunger and body (predetermined leakage)

Other lash compensators

Metering device

Metering valve meters the oil flow to the pushrod

Timing sets

Gear sets• Cam and crank rotate in opposite directions• Noisy if not free of burrs• Helical and spur cut gears

Timing sets

Timing chains• Single and double roller• Tensioners

Timing sets

Timing belts• Require maintenance• Quiet

Camshaft terminology

Cam lift (A-B)Valve lift = Cam lift times rocker ratio

Valve lift.300” cam lift times1.5 rocker ratio = .450” valve opening

Engine oiling

Lubrication through pressure. . .

Engine oiling

and spray. . .

Engine oiling

Oil pan baffles• To keep oil in sump during braking,

accelerating, and cornering

Engine oiling

Oil pan windage tray• To prevent oil aeration in the sump

Engine oiling

Oil pumps• Driven by distributors, gear on camshaft, or crankshaft

Engine oiling

Oil pumps with pressure relief valves• Gear type pump• Rotor type pump

Engine oiling

Full flow oil filtering system• Oil pump output flows through filter first• Bypass circuit for restricted filters will allow oil to flow to engine

Engine oils

API, SAE, and ASTM“S” - Spark ignition“C” - Compression ignition

Engine oil additives

Viscosity index improvers• To reduce viscosity change with heat

Detergents• To dissolve varnish and sludge

Dispersants• To keep sludge, carbon and other materials from recombining and suspends them in oil to be drained

Scuff inhibitors• To reduce friction and wear

Antifoam and antioxidants• To prevent foaming and to slow oxidation in oil

Engine measurements

Bore • Diameter of cylinder

Stroke• Distance between TDC & BDC

Engine measurements

Displacement per cylinder• r² S

Displacement for the engine• Disp per cylinder times the Number of cylinders

Engine measurements

Compression ratioD + CV

CV

To calculate clearance volume D . CR-1

Engine measurements

Deck clearance• Top of piston to top of block deck• Measured with dial indicator or depth mic

Engine measurements

Deck height• Center line of crank to block deck

Fits and clearances

Running fit• Clearance between bearing and shaft• Clearance for oil• Listed as diametral

Fits and clearances

Interference (press) fit• OD is larger than ID• Example is piston pin pressed into rod

Fits and clearances of pistons

Full floating• .0003 - .0005 clearance in rod• .0001 - .0003 clearance in piston

Oscillating• .0008 - .0012 interference in rod• .0003 - .0005 clearance in piston

Rod offset• Beam offset to center of cylinder• Enlarged chamfers to clear fillets

Pin offset• Offset to major thrust side • Quieter engine, less cylinder wear

Cooling system operation

Heat energy• 1/3 usable power• 1/3 released through exhaust system• 1/3 released through cooling system

Engine temperature• Cool enough to prevent part failure• Warm enough to maximize engine efficiency

Cooling system operation

Engine heat is transfered . . .• through walls of the combustion chambers• through the walls of cylinders

Coolant flows . . .• to upper radiator hose• through radiator• to water pump• through engine water jackets• through thermostat• back to radiator

Cooling system operation

Fans increase air flow through radiator• Hydraulic fan clutches• Hydraulic fans consume 6 to 8 HP• Electric fans

Coolant (ethylene glycol)• 50/50 mixture increases boiling point to 227°F• pressurizing system to 15 PSI increases to 265°F

Coolant (propylene glycol)• Less protection at the same temperatures• Less toxic

Combustion efficiency

Under perfect conditions . . .• Only byproducts would be carbon dioxide and water• Iso-octane fuel is laboratory fuel

Because conditions are not perfect . . .• Carbon monoxide and hydrocarbons are produced• Oxides of nitrogen are produced from pressure & temp

Emission controls• Catalytic converters – Convert CO & HC to carbon dioxide & water• O2 sensors – To monitor oxygen content in exhaust• EGR – To reduce peak cylinder temperatures